Display device color channel reconstruction

ABSTRACT

In general, the invention relates to techniques for reconstructing color channels in a multi-channel display device. The invention may be particularly useful in reconstructing the light source spectra for the color channels of liquid crystal displays (LCD). In order to accurately model and calibrate a display device, an accurate light source spectrum estimate for each of the individual color channels is needed. In accordance with the invention, a light source spectrum can be determined for each color channel of a display based on measured emission spectra for the color channels, an inverted contrast ratio for the display, and an assumed transmission spectrum for a light valve in the display. The invention provides techniques to compensate for light leakage from adjacent color channels with regards to wavelength dependent transmissions that cause hue shifts in images reconstructed by the display device.

TECHNICAL FIELD

The invention relates to color imaging and, more particularly, topresentation of color images on display devices.

BACKGROUND

Color display devices are typically multi-channel devices in the sensethat multiple physical color channels represent every pixel on thedisplay. Multi-channel display devices include cathode ray tubes (CRT),liquid crystal displays (LCD), plasma displays, and other imagingdevices. One common example of a multi-channel device is a three channeldevice comprising red, green, and blue (RGB) channels.

Each of the color channels in a multi-channel display device may bemodeled as a combination of a light source and a light valve. In thecase of the LCD, the light source typically comprises a common backlightand color filters for each of the channels. The light valve, in the caseof an LCD, comprises one or two fixed polarizers and liquid crystalcells (LCC), which rotate a polarization plane of passing light toregulate the amount of light emitted from the display.

An emission spectrum for the light source is useful for spectralmodeling and display calibration to improve color accuracy of imagerypresented by the display. However, the light source emission spectrum isusually unknown, partially because the spectra of light sources varybetween different manufacturers and models of display devices and fromdevice to device. Further, the emission spectrum for a light source maychange over time due to component aging, especially in the case of anLCD, which uses luminescent lamps as the common backlight.

SUMMARY

In general, the invention relates to techniques for reconstructing colorchannels in a multi-channel display device. The invention may beparticularly useful in reconstructing the light source spectra for thecolor channels of liquid crystal displays (LCD). In order to accuratelymodel and calibrate a display device, an accurate light source spectrumestimate for each of the individual color channels is needed.

In accordance with the invention, a light source spectrum can bedetermined for each color channel of a display based on measuredemission spectra for the color channels, an inverted contrast ratio forthe display, and an assumed transmission spectrum for a light valve inthe display. The invention provides techniques to compensate for lightleakage from adjacent color channels with regard to wavelength dependenttransmissions that cause hue shifts in images presented by the displaydevice.

In one embodiment, the invention is directed to a method comprisingmeasuring a first emission spectrum of a display for a maximum displaylevel, measuring a second emission spectrum of the display for a minimumdisplay level, and measuring cumulative emission spectra for each of aplurality of color channels of the display with the respective colorchannel at a maximum level and the other channels at minimum levels. Themethod further includes assuming a transmission spectrum for a lightvalve in the display operating at a maximum level, and determining aninverted contrast ratio based on the first emission spectrum measurementand the second emission spectrum measurement. A set of equations iscreated for the color channels based on the measured cumulative emissionspectra for the color channels, the measured inverted contrast ratio,and the assumed transmission spectrum. The equation set is then solvedto determine a light source spectrum for each of the color channels.

In another embodiment, the invention is directed to a system including adisplay, a plurality of color channels in the display, a light sourceand a light valve to model each of the color channels, and means fordriving the light valve based on a color profile defined by light sourcespectra. The light source spectra are reconstructed from measuredemission spectra for the color channels, an inverted contrast ratio forthe display, and an assumed transmission spectrum for the light valve inthe display.

In a further embodiment, the invention is directed to a method whichdetermines a light source spectrum for each of a plurality of colorchannels of a display based on measured emission spectra for the colorchannels, an inverted contrast ratio for the display, and an assumedtransmission spectrum for a light valve in the display.

In an added embodiment, the invention is directed to a computer-readablemedium containing instructions. The instructions cause a programmableprocessor to receive cumulative emission spectrum measurements for eachof a plurality of color channels of a display with the respective colorchannel at a maximum level and the other channels at minimum levels,assume a transmission spectrum for a light valve in the displayoperating at a maximum level, and determine an inverted contrast ratiofor the display. The computer-readable medium contains furtherinstructions that cause the programmable processor to solve a set ofequations to determine a light source spectrum for each of the colorchannels based on the measured cumulative emission spectra for the colorchannels, the inverted contrast ratio, and the assumed transmissionspectrum, and drive the light valve based on a color profile defined bythe light source spectra.

In another embodiment, the invention is directed to a method comprisingmeasuring cumulative emission spectra for each of a plurality of colorchannels of a display with the respective color channel at a maximumlevel and the other channels at minimum levels, assuming a firsttransmission spectrum for a light valve in the display operating at amaximum level, and assuming a second transmission spectrum for the lightvalve in the display operating at a minimum level. The method furthercomprises calculating an inverted contrast ratio based on the firsttransmission spectrum assumption and the second transmission spectrumassumption, creating a set of equations for the color channels based onthe measured cumulative emission spectra for the color channels, theinverted contrast ratio, and the assumed first transmission spectrum,and solving the equations to determine a light source spectrum for eachof the color channels.

The invention is capable of providing many advantages. The describedembodiments can improve color accuracy, and reduce color accuracyvariation for images presented by different types and brands of displaydevices. In display devices such as LCDs, for example, the light valvetransmission spectrum is dependent on a wavelength and a digital drivingsignal. The described embodiments take the wavelength dependency and itssubsequent effects on the light source spectrum into account. Forexample, the color channels cannot achieve a fully closed state, evenwhen set at a minimum level, due to transmission spectrum wavelengthdependency. The emission spectra measurements for each of the maximumlevel color channels include light leakage from the minimum level,adjacent color channels. The light source emission spectra measurementaccuracy improves due to compensation of the excess light emission. Suchcompensation allows for the more accurate calibration of a displaydevice color model and reduces non-physical effects in model calibrationcaused by contamination of the measurements by light leakage. Thecapability of reconstructing the light source spectra adds flexibilityto color applications and allows for less dependency on particular typesand brands of display devices to present imagery with consistent colorquality.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram illustrating a color management system thatmakes use of color profiles formulated for a display device inaccordance with the invention.

FIG. 2 is a block diagram illustrating a color profile generation systemthat generates a color profile based on a display device color model.

FIG. 3 is a block diagram illustrating a light source reconstructionsystem that reconstructs light source spectra based on display devicespectral emission measurements.

FIG. 4 is a schematic diagram illustrating a color channel of amulti-channel liquid crystal display (LCD) device as a portion of thelight source reconstruction system from FIG. 3.

FIG. 5 is a flow diagram illustrating a method for reconstructing lightsource emission spectra for the multi-channel liquid crystal display.

FIG. 6 is a flow diagram illustrating another method for reconstructinglight source emission spectra for the multi-channel liquid crystaldisplay.

DETAILED DESCRIPTION

FIG. 1 is a block diagram illustrating a color management system 10 thatmakes use of color profiles formulated for a display device inaccordance with the invention. As will be described, the color profilesare built based on a display device color model that makes use of alight source spectrum reconstruction for a multi-channel color displaydevice that takes into account light leakage from adjacent colorchannels and wavelength dependent transmissions. The wavelengthdependency causes hue shifts in the images reconstructed by the displaydevice, as discussed in Reflective Liquid Crystal Displays by Wu S. andYang D., John Wiley & Sons Ltd, p. 335, 2001. By taking into accountlight leakage from adjacent color channels and wavelength dependenttransmissions, the color profiles used by color management system 10promote increased color image accuracy between images produced ondifferent multi-channel color display devices.

As shown in FIG. 1, color management system 10 includes a colormanagement module 12 that generates a color map 18 between a sourcedevice 13 and a destination device 24 based on a source color profile 14and a destination color profile 16. Color map 18 defines a conversionbetween source coordinates 20 associated with source device 13 anddestination coordinates 22 associated with destination device 24.Destination device 24 may be a multi-channel color display devicesincluding a liquid crystal display (LCD), a cathode ray tube (CRT)display, a plasma display, or the like. In some embodiments, sourcedevice 13 may be a scanner, a camera, or the like that acquires animage. An original image obtained by source device 13 is color correctedby color management module 12 using color map 18 prior to beingdisplayed via the destination display device 24. In another embodiment,a color management module may color correct an original image of asource device without generating a color map by using a combination ofsource and destination color profiles.

Color management module 12 may be realized by one or more softwareprocesses executing on a processor such as a desktop computer orworkstation. Module 12 executes computer-readable instructions tosupport, at least in part, the functionality described herein. Colormanagement module 12 facilitates color matching between destinationdevice 24 and source device 13. Source color profile 14 specifies a setof color response characteristics associated with source device 13.Destination color profile 16 specifies a set of color responsecharacteristics associated with destination device 24.

Source and destination color profiles 14, 16 permit reconciliation ofcolor response differences between source device 13 and destinationdevice 24 so that an image obtained by source device 13 can beaccurately represented on destination display device 24. Source anddestination color profiles 14, 16 may generally conform to profilesspecified by the International Color Consortium (ICC). Sourcecoordinates 20 specify color image values for an image in adevice-dependent coordinate system associated with source device 13,e.g., RGB in the event source device 13 is a scanner. Destinationcoordinates 22 specify color image values for an image in adevice-dependent coordinate system associated with destination device24.

FIG. 2 is a block diagram illustrating a color profile generation system30 according to an embodiment of the invention. Within system 30, aprofile generation module 40 generates a color profile 42 based on dataobtained from a display device, such as destination display device 24 ofFIG. 1. The data obtained from the display device include light sourcespectra 34 and device coordinates 36, which are representative of theoutput characteristics of light sources and light valves, respectively,which form part of the display device. A device color model 32 useslight source spectra 34 and device coordinates 36 to generate colorcoordinates 38. Profile generation module 40 builds color profile 42based on the relationship between device coordinates 36 and colorcoordinates 38. In the case illustrated in FIG. 2, light source spectra34 are included to improve calibration accuracy of device color model 32and therefore, improve accuracy of color coordinates 38 and colorprofile 42. In some embodiments, a device color model does not inputlight source spectra and generates color coordinates based only ondevice coordinates, e.g. RGB.

In other embodiments, a color profile is built without a device colormodel; however, an accurate color profile would require a significantnumber of measurements. Building and calibrating a device color modelreduces the number of measurements needed to build a color profile as noadditional measurements are needed after the device color model iscalibrated. The device color model provides a response similar to adisplay device, but generates device-independent color coordinates.

Profile generation module 40 sends device coordinates 36 to device colormodel 32 and receives a modeled response in the form of colorcoordinates 38. Profile generation module 40 then creates color profile42 that is capable of converting images from device-independent colorcoordinates to device specific coordinates. Profile generation module 40may be configured to control device coordinates 36, and therefore thelight valves in the display device to obtain color coordinates 38 fromdevice color model 32, and generate color profile 42 based on thereceived data. As will be described, light source spectra 34 arecalculated from equations based on measured and assumed spectralemissions of the display device.

The display device may comprise any number of color channels, but forpurposes of illustration, will be described herein as a three-channeldisplay system with a red channel, a green channel, and a blue channel(RGB). Each pixel of an exemplary display device, such as an LCD,includes three color channels that combine to generate the color neededto accurately reproduce a pixel of an image. Each of the color channelsincludes one of the light sources and one of the light valves that maybe manipulated to achieve the color specified for the pixel. The lightsource emission spectrum 34 S_(i)(λ) of the i^(th) channel is a functionof the wavelength, λ, and determines the color of the channel. The lightsource comprises a combination of a backlight B(λ) and a color filterF_(i)(λ) for the i^(th) color channel, as folows:S _(i)(λ)=B(λ)*F _(i)(λ)  (1)The light valve transmission spectrum φ(d, λ) is controlled by devicecoordinates 36 and may be assumed to be identical for every colorchannel. In a typical LCD device, stationary polarizers and liquidcrystal cells (LCC) with controllable phase retardations constitute thelight valves. A voltage applied to the LCC is dependent upon the digitaldriving signal d and determines the phase retardation for passing light.For polarized light, the phase retardation controls rotation of apolarization plane of the light and therefore, the wavelength dependentintensity of light transmitted through the light valves. An emissionspectrum for an individual color channel i and for a given digital leveld may be expressed as a product of the light source emission spectrumS_(i) (λ) and the light valve transmission spectrum φ(d, λ) as follows:E _(i)(d,λ)=S _(i)(λ)*φ(d,λ)  (2)A cumulative emission spectrum for a pixel is a summation of all Nindividual color channels, in this case the red, green, and blue colorchannels. $\begin{matrix}{{E\left( {d_{r},d_{g},d_{b},\lambda} \right)} = {{\sum\limits_{i = 1}^{N}\quad E_{i}} = {{{S_{r}(\lambda)}*{\phi\left( {d_{r},\lambda} \right)}} + {{S_{g}(\lambda)}*{\phi\left( {d_{g},\lambda} \right)}} + {{S_{b}(\lambda)}*{\phi\left( {d_{b},\lambda} \right)}}}}} & (3)\end{matrix}$The light sources determine the color of light emitted for each of thecolor channels. The digital driving signal d controls the intensity oflight transmitted by the light valves for each of the color channels.

In accordance with the invention, profile generation module 40 generatescolor profile 42 for the LCD based on color coordinates 38, which aredetermined by device color model 32 from light source spectra 34 anddevice coordinates 36. The resulting color profile 42 represents thecolor response characteristics of the LCD device. In order to accuratelygenerate color profile 42 for any type or brand of LCD, light sourcespectra 34 must be determined from display measurements andcalculations. The calculations compensate for adjacent channel leakageand light valve wavelength dependency that create inaccuracies in directlight source spectral measurements. The compensated light source spectra34 improve calibration of device color model 32 and therefore create amore accurate color profile 42 for the display device. Profilegeneration module 40 may be realized by one or more software processesexecuting on a processor such as a desktop computer or workstation.

FIG. 3 is a block diagram illustrating a light source reconstructionsystem 50 that generates light source spectra 34 from display devicespectral emission measurements. In the example of FIG. 3, the spectralemission measurements are from the three-channel LCD described above.System 50 includes color channel emission spectra 54, an assumedtransmission spectrum 56, a maximum emission spectrum 58, and a minimumemission spectrum 60 input to a light source reconstruction module 52.Light source reconstruction module 52 makes use of an inverted contrastratio 62 and an equation solver module 64. Equation solver module 64generates light source spectra 34 based on color channel emissionspectra 54, assumed transmission spectrum 56, and inverted contrastratio 62. In particular, light source spectra 34 are determined bysolving equations based on the measured color channel emission spectra54, assumed transmission spectrum 56, and inverted contrast ratio 62.

The measured emission spectra of the display for the red, green, andblue channels are included in color channel emission spectra 54. Colorchannel emission spectra 54 comprise cumulative emission measurements ofeach color channel with the respective color channel at a maximum leveland the other channels at minimum levels. For example, the red channelemission spectrum comprises the cumulative emission measurement of thedisplay when the red channel is at the maximum digital driving signaland the green channel and blue channel are at the minimum digitaldriving signal. The green channel and blue channel minimum levelemissions cannot be assumed negligible, however, and can generatesignificant emission even when the pertinent light valves are turned“off.” In particular, the minimum level light valve transmissionspectrum φ(0,λ) is still dependent on wavelength. Consequently, thelight valves cannot fully close to block all light from being emitted.The resulting emissions for the red (255,0,0,λ), green (0,255,0,λ) andblue (0,0,255,λ) channels, at maximum 8-bit (255) drive levels withadjacent channels at minimum drive levels, are represented as follows:Ê(255,0,0, λ)=S _(r)(λ)*φ(255,λ)+S _(g)(λ)*φ(0,λ)+S _(b)(λ)*φ(0,λ){circumflex over (E)}(0,255,0,λ)=S _(r)(λ)*φ(0,λ)+S _(g)(λ)*φ(255,λ)+S_(b)(λ)*φ(0,λ)Ê(0,0,255,λ)=S _(r)(λ)*φ(0,λ)+S _(g)(λ)*φ(0,λ)+S _(b)(λ)*φ(255,λ)  (4)where E is emission, S is the spectral contribution of light source, andφ(255,λ) is the digital driving value for a given light valve. Theassumed transmission spectrum 56 is determined for the maximum digitaldriving signal, φ(255,λ). In this case, the maximum digital drivingsignal represents an 8-bit system for purposes of example. Of course,other n-bit systems are possible. The determination may be made byassuming the measured color channel emission spectra 54 constitute thelight source spectra for the color channels, using default parametersfor a particular type or brand of LCD, or squaring a cosine or sinefunction of phase retardation associated with the light valves. The mainfunction of assumed transmission spectrum 56 is to normalize themeasured color channel emission spectra 54 and compensate for thewavelength dependency of the light valves.

Inverted contrast ratio 62 may be based on the maximum leveltransmission spectrum φ(255,λ) and the minimum level transmissionspectrum φ(0,λ) for a given channel. Although both of the transmissionspectrum values may be assumed, as described for assumed transmissionspectrum 56, the uncertainty of the rough approximations may introducesignificant error in light source spectra 34, which is perceived as ahue shift in color profile 42. In the embodiment illustrated in FIG. 3,a measured contrast ratio, Ĉ, is used in place of the assumed ratio.Maximum emission spectrum 58 of the display is measured for all of thecolor channels in the display at the maximum level, and minimum emissionspectrum 60 is measured for all the color channels at the minimum level.As can be seen, the inverted ratio of emission spectrum measurements 58and 60 is equivalent to the assumed transmission spectrum ratio.$\begin{matrix}{\hat{C} = {\frac{\hat{E}\left( {0,0,0,\lambda} \right)}{\hat{E}\left( {255,255,255,\lambda} \right)} = {\frac{\left( {{S_{r}(\lambda)} + {S_{g}(\lambda)} + {S_{b}(\lambda)}} \right)*{\phi\left( {0,\lambda} \right)}}{\left( {{S_{r}(\lambda)} + {S_{g}(\lambda)} + {S_{b}(\lambda)}} \right)*{\phi\left( {255,\lambda} \right)}} = \frac{\phi\left( {0,\lambda} \right)}{\phi\left( {255,\lambda} \right)}}}} & (5)\end{matrix}$

As a result, equations for the reconstruction of all channels includeonly one assumed variable and produce a converging set of equations thatcan be solved by iteration. Accordingly, light source reconstructionmodule 52 applies the input values to equation solver module 64.Equation solver module 64 creates a light source emission equation foreach of the color channels. For the case in which inverted contrastratio 62 is based on the measured emission spectra for maximum andminimum display levels, as in equation (5), the light source spectrumequations may be expressed as: $\begin{matrix}{{{S_{r}(\lambda)} = {\frac{\hat{E}\left( {255,0,0,\lambda} \right)}{\phi\left( {255,\lambda} \right)} - {\left( {{S_{g}(\lambda)} + {S_{b}(\lambda)}} \right)*{\hat{C}(\lambda)}}}}{{S_{g}(\lambda)} = {\frac{\hat{E}\left( {0,255,0,\lambda} \right)}{\phi\left( {255,\lambda} \right)} - {\left( {{S_{r}(\lambda)} + {S_{b}(\lambda)}} \right)*{\hat{C}(\lambda)}}}}{{S_{b}(\lambda)} = {\frac{\hat{E}\left( {0,0,255,\lambda} \right)}{\phi\left( {255,\lambda} \right)} - {\left( {{S_{r}(\lambda)} + {S_{g}(\lambda)}} \right)*{\hat{C}(\lambda)}}}}} & (6)\end{matrix}$The first members of equations (6) are normalized on the assumed maximumlight valve transmission 56. The normalization compensates wavelengthdependency of the transmission spectrum. The second members of equations(6) are light leakage compensation that, for example, model the excesslight passed through the minimum level green and blue channels whileoperating the red channel at the maximum level.

Equation solver module 64 solves the light source spectrum equations,(6). Equation set (6) converges, which allows equation solver module 64to solve the set by iterations. A high inverted contrast ratio 62ensures that equation set (6) will converge. The outputs of equationsolver module 64 are reconstructed light source emission spectra 34.Light source emission spectra 34 include light source spectra for eachof the color channels in the LCD device. The light source spectra 34improve calibration accuracy of device color model 32 to accuratelymodel the display device and generate color profile 42, as shown in FIG.2. System 50 improves the color accuracy of the multi-channel LCD bycompensating light leakage contamination in light source spectralmeasurements.

FIG. 4 is a schematic diagram illustrating a color channel 80 of atypical liquid crystal display (LCD) device as a portion of light sourcereconstruction system 50, from FIG. 3. Color channel 80 includes a lightsource 81 and a light valve 85. Light source 81 includes a backlight 82and a color filter 84. Light valve 85 includes a first polarizer 86, asecond polarizer 88, and liquid crystal cells (LCC) 90 disposed betweenthe polarizers 86 and 88.

Backlight 82 emits light to every pixel, and therefore every colorchannel 80. First polarizer 86 of light valve 85 polarizes the passinglight from backlight 82. LCC 90 rotates the polarization plane of thepassing light. The amount of light transmitted by light valve 85 dependson an orientation of the polarization plane of the passing lightrelative to second polarizer 88. An angle of rotation of thepolarization plane depends on a voltage or digital driving signalapplied to LCC 90 and a wavelength of the light. Color filter 84 filtersthe light transmitted by light valve 85 to define the color of channel80. In the case of the three-channel LCD described above, color filter84 may be a red, green, or blue filter.

Light source reconstruction system 50, from FIG. 3, may improve thecolor accuracy of images displayed by the LCD. System 50 uses displayemission measurements to create and calibrate display device color model32, which is then used to create color profile 42 for the display. Colorprofile 42 drives light valve 85, e.g., from a host computer coupled toa destination display device, to generate a precise color output fromcolor channel 80 and the LCD. Display device emission may vary betweentypes and brands of displays. The color profile 42 built by colorprofile generation system 30 from FIG. 2 may allow any LCD device tomore accurately present the intended color of an image obtained by asource device.

FIG. 5 is a flow diagram illustrating a method for reconstructing lightsource emission spectra 34 for the multi-channel liquid crystal displaydescribed above. Light source reconstruction system 50 from FIG. 3reconstructs the light source spectra 34 to create and calibrate devicecolor model 32. Device color model 32 may model the LCD and generatecolor coordinates 38 used to build color profile 42 for the LCD.

Measurements and assumptions from the display device are used togenerate light source spectra 34. Maximum emission spectrum 58 ismeasured (100) for all color channels 80 set at a maximum level, whichmay also be considered an emission of white light. Minimum emissionspectrum 60 is then measured (102) for all of the color channels 80 setat a minimum level, which may be considered a black emission. Cumulativeemission spectra 54 are measured for each of the individual colorchannels 80 (104) with the respective channel at a maximum level and theother channels at minimum levels.

Assumed transmission spectrum 56 is assumed for the light valves 85within the LCD operating at a maximum level (106). Assumed transmissionspectrum 56 is known with accuracy between 5% and 20%. One of theassumption methods discussed in reference to FIG. 3 may be used todetermine the assumption. Inverted contrast ratio 62 is determined (108)based on the maximum and minimum emission spectrum measurements 58 and60.

The color channel emission spectra 54, assumed transmission spectrum 56,and measured inverted contrast ratio 62 are used by equation solvermodule 64 in light source reconstruction module 52 to create a set ofequations for light source emission spectra of the color channels 80(110). The set of equations is then solved by equation solver module 64for each of the light source spectra 34 (112).

Accurate light source emission spectra 34 are very important forcalibration of spectral models. Optimization of a spectral model by themethod illustrated in FIG. 5 results in per-channel accuracy of lessthan ΔE=4. Although the human eye can perceive hue shifts greater thanΔE=3, the color reconstruction method far surpasses the shift of ΔE=16experienced when no reconstruction method is applied to the lightsources 81.

FIG. 6 is a flow diagram illustrating another method for reconstructinglight source emission spectra 34 for the multi-channel liquid crystaldisplay described above. Device color model 32 uses the light sourceemission spectra 34 to model and calibrate the display. The cumulativeemission spectrum 54 is measured for each of the color channels 80 (120)with the respective channel at a maximum level and the other channels atminimum levels. Transmission spectra are assumed to be known for thelight valves within the color channels 80. Assumed transmission spectrum56 is assumed for one of the light valves operating at a maximum level(122).

A minimum transmission spectrum is assumed for one of the light valveoperating at a minimum level (124). The assumed transmission spectra areknown with accuracy between 5% and 20%. One of the methods discussed inreference to FIG. 3 may be used to determine the assumptions. Theinverted contrast ratio 62 is calculated (126) based on assumed maximumtransmission spectrum 56 and the assumed minimum transmission spectrum.

Color channel emission spectra 54, assumed maximum transmission spectrum56, and calculated inverted contrast ratio 62 are applied to equationsolver module 64 within light source reconstruction module 52 to createa set of equations for the light source emission spectra of the colorchannels 80 (128). The set of equations is then solved by equationsolver module 64 for each of light source spectra 34 (130).

The method illustrated in FIG. 6 results in a reduced hue shift comparedto the shift of ΔE=16 experienced when no reconstruction method isapplied to the light sources. However, the assumed inverted contrastratio causes a larger hue shift to occur than the measured invertedcontrast ratio used in the method illustrated in FIG. 5.

Various embodiments of the invention have been described. These andother embodiments are within the scope of the following claims.

1. A method comprising: measuring a first emission spectrum of a display for a maximum display level; measuring a second emission spectrum of the display for a minimum display level; measuring cumulative emission spectra for each of a plurality of color channels of a display with the respective color channel at a maximum level and the other channels at minimum levels; assuming a transmission spectrum for a light valve in the display operating at a maximum level; determining an inverted contrast ratio based on the first emission spectrum measurement and the second emission spectrum measurement; creating a set of equations for the color channels based on the measured cumulative emission spectra for the color channels, the measured inverted contrast ratio, and the assumed transmission spectrum; and solving the equations to determine a light source spectrum for each of the color channels.
 2. The method of claim 1, wherein the cumulative emission spectrum of the display comprises a summation of all color channel emission spectra.
 3. The method of claim 2, wherein the emission spectrum for each of the color channels combines the light source spectrum for the color channel and the transmission spectrum for the light valve.
 4. The method of claim 1, wherein the light source spectrum for each of the color channels comprises a backlight spectrum and a transmittance spectrum for a filter of each color channel.
 5. The method of claim 1, wherein the transmission spectrum is dependent upon a digital driving signal and the wavelength of the light source.
 6. The method of claim 1, wherein assuming the transmission spectrum for the light valve includes at least one of: assuming the cumulative emission spectrum for the respective color channel constitutes the light source spectrum for the color channel; using default parameters for a particular type of the display; squaring a cosine function of a phase retardation associated with the light valve; and squaring a sine function of the phase retardation associated with the light valve.
 7. The method of claim 1, wherein the first emission spectrum measurement comprises all the color channels operating at a maximum digital driving signal to generate a white display.
 8. The method of claim 1, wherein the second emission spectrum measurement comprises all the color channels operating at a minimum digital driving signal to generate a black display.
 9. The method of claim 1, wherein the plurality of color channels comprises a red channel, a green channel, and a blue channel.
 10. The method of claim 1, wherein the display comprises a liquid crystal display (LCD).
 11. A multi-channel display system comprising: a display; a plurality of color channels in the display; a light source and a light valve to model each of the color channels; and means for driving the light valve based on a color profile defined by light source spectra, the light source spectra reconstructed from measured emission spectra for the color channels, an inverted contrast ratio for the display, and an assumed transmission spectrum for the light valve in the display.
 12. The multi-channel display system of claim 11, wherein the display comprises a liquid crystal display (LCD).
 13. The multi-channel display system of claim 11, wherein the light source comprises a backlight and color filters.
 14. The multi-channel display system of claim 11, wherein the light valve comprises fixed polarizers and rotating liquid crystal cells (LCC).
 15. The multi-channel display system of claim 14, wherein the LCC rotation depends on a wavelength of the light source and a digital driving signal.
 16. The multi-channel display system of claim 11, wherein the plurality of color channels comprise a red channel, a green channel, and a blue channel.
 17. The multi-channel display system of claim 11, wherein the driving means sets a digital driving signal of the light valve based on the color profile.
 18. A method comprising determining a light source spectrum for each of a plurality of color channels of a display based on measured emission spectra for the color channels, an inverted contrast ratio for the display, and an assumed transmission spectrum for a light valve in the display.
 19. The method of claim 18, wherein the measured emission spectra for the color channels comprise a respective color channel at a maximum level and the other channels at minimum levels.
 20. The method of claim 18, wherein the inverted contrast ratio for the display comprises measured emission spectra for a maximum display level and a minimum display level.
 21. The method of claim 18, wherein the inverted contrast ratio for the display comprises assumed transmission spectra for the light valve operating at a maximum level and a minimum level.
 22. The method of claim 18, wherein the assumed transmission spectrum for the light valve is at a maximum level.
 23. The method of claim 18, wherein assuming the transmission spectrum for the light valve includes at least one of: assuming the measured emission spectra for the color channels constitute the light source spectra for the color channels; using default parameters for a particular type of the display; squaring a cosine function of a phase retardation associated with the light valve; and squaring a sine function of the phase retardation associated with the light valve.
 24. The method of claim 18, wherein the plurality of color channels comprises a red channel, a green channel, and a blue channel.
 25. The method of claim 18, wherein the display comprises a liquid crystal display (LCD).
 26. A computer-readable medium comprising instructions for causing a programmable processor to: receive cumulative emission spectrum measurements for each of a plurality of color channels of a display with the respective color channel at a maximum level and the other channels at minimum levels; assume a transmission spectrum for a light valve in the display operating at a maximum level; determine an inverted contrast ratio for the display; solve a set of equations to determine a light source spectrum for each of the color channels based on the measured cumulative emission spectra for the color channels, the inverted contrast ratio, and the assumed transmission spectrum; and drive the light valve based on a color profile defined by the light source spectra.
 27. The computer-readable medium of claim 26, wherein the inverted contrast ratio of the display is based on a measured first emission spectrum of the display for a maximum display level and a measured second emission spectrum of the display for a minimum display level.
 28. The computer-readable medium of claim 26, wherein the inverted contrast ratio of the display is based on the assumed transmission spectrum for the light valve in the display operating at the maximum level and an assumed transmission spectrum for the light valve in the display operating at a minimum level.
 29. The computer-readable medium of claim 26, wherein the instructions for causing a programmable processor to assume the transmission spectrum for the light valve includes at least one of instructions for causing a programmable processor to: assume the cumulative emission spectrum for the respective color channel constitutes the light source spectrum for the color channel; use default parameters for a particular type of the display; square a cosine function of a phase retardation associated with the light valve; and square a sine function of the phase retardation associated with the light valve.
 30. The computer-readable medium of claim 26, wherein the instructions for causing a programmable processor to drive the light valve comprise instructions for causing a programmable processor to set a digital driving signal of the light valve based on the color profile.
 31. A method comprising: measuring cumulative emission spectra for each of a plurality of color channels of a display with the respective color channel at a maximum level and the other channels at minimum levels; assuming a first transmission spectrum for a light valve in the display operating at a maximum level; assuming a second transmission spectrum for the light valve in the display operating at a minimum level; calculating an inverted contrast ratio based on the first transmission spectrum assumption and the second transmission spectrum assumption; creating a set of equations for the color channels based on the measured cumulative emission spectra for the color channels, the calculated inverted contrast ratio, and the assumed first transmission spectrum; and solving the equations to determine a light source spectrum for each of the color channels.
 32. The method of claim 31, wherein the cumulative emission spectrum of the display comprises a summation of all color channel emission spectra.
 33. The method of claim 32, wherein the emission spectrum for each of the color channels combines the light source spectrum for the color channel and the transmission spectrum for the light valve.
 34. The method of claim 31, wherein the light source spectrum for each of the color channels comprises a backlight spectrum and a transmittance spectrum for a filter of each color channel.
 35. The method of claim 31, wherein the first transmission spectrum assumption comprises the light valve operating at a maximum digital driving signal to allow a maximum amount of light to be emitted.
 36. The method of claim 31, wherein the second transmission spectrum assumption comprises the light valve operating at a minimum digital driving signal to allow a minimum amount of light to be emitted.
 37. The method of claim 31, wherein the plurality of color channels comprises a red channel, a green channel, and a blue channel.
 38. The method of claim 31, wherein the display comprises a liquid crystal display (LCD).
 39. The method of claim 31, wherein the transmission spectrum is dependent upon a digital driving signal and the wavelength of the light source.
 40. The method of claim 31, wherein assuming the transmission spectrum for the light valve includes at least one of: assuming the cumulative emission spectrum for the respective color channel constitutes the light source spectrum for the color channel; using default parameters for a particular type of the display; squaring a cosine function of a phase retardation associated with the light valve; and squaring a sine function of the phase retardation associated with the light valve. 